2. Multiple sidebars (a)

3. Multiple sidebars (b)

If your data is sorted into a useful order, you might prefer the data to be arranged
vertically.
You can distribute the entries into three vertical columns by dividing the entry number by a third of the total number of markers.
You need to take a bit of care with Math.ceil() and Math.floor() to avoid the possibility of the final marker being placed in
the next column.